The present invention relates generally to optical safety filters. More specifically, the present invention relates to optical safety filters that provide infrared and near-infrared filtration that can be applied in a variety of environments for protection in the form of solar radiation shields, heat shields and laser emission filters.
It is well known that a number of different applications require effective shielding against exposure to infrared and near infrared radiation emissions. To protect against the damage caused and heat gain attributed to infrared and near infrared emissions in the context of solar radiation, various prior art films have been applied to structural and/or automotive windows to reduce glare and to provide solar screening. For example, plastic films that are dyed or coated to provide desired optical properties are applied to the interior surfaces of such windows. Typically such a film that provides effective solar screening is one that, through energy absorbtion, has a low transmission in both the visible range (400 to 700 nm) and the infrared and near infrared range (700 to 3000 nm). However, these prior art dyed films are soft and succeptible to damage, scratching and peeling. Further, the dyes in the films often fade with solar exposure. Also, when the films are colored with multiple dyes, the resulting film is a muddy green color wherein the dyes often fade at different rates, causing unwanted color changes over the life of the film.
Other known infrared and near infrared filteration windows are fabricated using vacuum-deposited grey metals, such as stainless steel, inconel, monel, chrome or nichrome alloys. The deposited grey metal films offer about the same degrees of transmission in the visible and near infrared portions of the solar spectrum. As a result, the grey metal films are an improvement over dyed films with regard to solar control. The grey metal films are relatively stable when exposed to light, oxygen and moisture, and in those cases in which the transmission of the coatings increases due to oxidation, color changes are generally not detectable. After application to clear float glass, grey metals block light transmission by approximately equal amounts of solar reflection and solar absorption.
Other vacuum-deposited metal layers such as silver, gold, aluminum and copper control solar radiation primarily by reflection. Because of the high reflection in the visible spectrum (i.e., high R.sub.VIS), films having these vacuum-deposited layers are useful in only a limited number of applications. A modest degree of selectivity of transmission in the visible spectrum over transmission in the near infrared spectrum is afforded by certain reflective materials, such as copper and silver. A particular difficulty in this application is that the cost of materials and the finished filter tend to be very expensive.
In other contexts, there are a number of commercial and military fields where there is a growing awareness that certain wavelengths of energy emissions are harmful to the eye. Generally, such energy emissions, in the form of a laser emission, are grouped at or near the infrared range corresponding to approximately 750-3000 nm. For example, energy emitted from a laser operating in this wavelength range can cause both temporary and permanent blindness and can be disorienting to those people that have been exposed. The adverse effects of energy emissions having a wavelength within this wavelength region are only recently beginning to be fully recognized as applications that utilize such energy emissions are more frequently employed. For example, there are a number of optical communication protocols that utilize lasers tuned to these wavelengths for the transmission of data as well as a number of military applications that employ infra-red and near infra-red laser energy emissions at these wavelengths in connection with the sighting of weapons and target acquisition. Further, many industries are beginning to employ laser cutting systems that employ infrared/near-infrared lasers. As the environments in which the use of such energy emissions increases, the potential for accidental exposure to such emissions also greatly increases.
In the past, to avoid accidental exposure to infra-red laser emissions, people have attempted to protect their eyes through the use of currently available optical filters that contain both narrow and broad band absorbers that block light over a wide range of wavelengths resulting in an overly dark filter that screens out the potential for exposure to harmful emission levels. In this regard, however, the broadband filters only reduced the magnitude of the exposure rather than screening out the harmful wavelengths of energy. As a result, with only a few exceptions, such filters have generally been directed toward the reduction in intensity of the light transmitted, rather than to the filtering of any particular wavelength or group of wavelengths.
The problem with such a prior art approach is that the nonselective reduction in overall light transitivity generally impacts the visual acuity of the wearer making the use of such filtering difficult if not impossible to implement due to the severe limitations imposed on the visibility of the wearer. One key area that further limits the wearability of such generalized filters is traffic signal recognition. To meet the standards required for use as sunglasses, the wearer must be able to differentiate between red and green traffic signals. Often broad filters directed at screening the above laser energy emissions also result in severely limiting the wearer's ability to differentiate between red and green objects making traffic identification difficult if not impossible.
Another prior art approach involved in laser filtering related to the use of specialty lenses. The difficulty with such lenses is that they typically have a limited range of properties, because they are made of glass or high impact polymers such as polycarbonate, thereby requiring that the additives used to modify the transmissivity must be compatible with the high temperatures required in making the glass or molding of the polymer material. For example, in forming a polycarbonate lens at molding temperatures of 550° F. the dyes implemented must have a very high thermal stability and must be added at relatively high concentrations to protect against their breakdown during the molding process.
As a result, the range of substances that are available that are both compatible with the high molding temperatures and capable of imparting the desired filtering properties is very narrow and generally does not provide the versatility typically encountered with organic dyestuffs that are normally utilized for narrow wavelength filtering. Such dyes are generally fully within the green range as the neutral color dyes break down at the temperature ranges discussed herein. The resulting filter that provides the desired level of protection typically employs a high quantity of greenish dye that produces a very undesirable muddy green color. Not only is this undesirable from a commercial standpoint, it further encounters the problem that the filter does not allow the wearer to differentiate well between reds and greens. Finally, such a lens has a low light transmissivity on the order of 24% because the overall quantity of dye required in making the lens is so high.
A similar issue arises when employing a coating method for providing infrared/near infrared filtration. The problem that arises with coatings is that the coating application is generally thin, on the order of 1-5 microns, making it very difficult to get sufficient filtering protection. When using the coating method to get to the desired level of protection, the dye concentration must be high creating a coating material that is very dark. These dark coatings in turn greatly reduce the VLT of the filter. In addition, most of IR/NIR dyes have very limited solubility in polar coatings for desired concentration to obtain protection.
As a result there is a need for an optical filter that blocks infrared/near infrared energy emissions, such as laser energy emissions, while preserving the wearer's ability to differentiate between reds and greens. There is a further need for an optical filter that includes neutral colored filtration dyes that filter laser energy emissions while having a pleasing overall color and while also having a high VLT on the order of 65%. There is still a further need for an optical filter that meets the requirements set forth to create an effective heat shield that blocks infrared/near infrared energy emissions while also maintaining the desired visible light transmission characteristics.